Answer to Stack Overflow question: http://stackoverflow.com/questions/37825116/how-to-update-new-plotline-values-instead-of-removing-and-adding-new-ones-in-hig

author(s): Mike Zavarello

HTML

<script src="https://code.highcharts.com/highcharts.js"></script>

<div id="container" style="height: 400px"></div>
<button id="button" class="autocompare">Update first point</button>

JavaScript

$(function () {
    $('#container').highcharts({
        chart: {
            type: 'line'
        },
        xAxis: {
            categories: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
        },

        series: [{
            data: [29.9, 71.5, 106.4, 129.2, 144.0, 176.0, 135.6, 148.5, 216.4, 194.1, 95.6, 54.4]
        }, {
        		// scatter plot line to serve as moveable plot line
            type: 'scatter', 
            data: [[0,10],[11,10]], 
            lineWidth: 1, 
            marker: {
            	enabled: true						// hide the markers (to show only the line)
            },
            showInLegend: true,				// hide in legend
            enableMouseTracking: false	// prevent user from interacting with this line
        }]
    });

    // button handler
    var chart = $('#container').highcharts(),
        y = 30;
    $('#button').click(function () {
        y += 10;
        // update both ends of the scatter line
        chart.series[1].data[0].update([0,y]);
        chart.series[1].data[1].update([11,y]);
    });
});